it may quite as often arise from the circumstance that such accumulations of loose ejected materials have been exposed from the first to greater waste in regions where the snows melt suddenly, and where the winds are most violent. A dense covering of turf and shrubs, the most effective of all preservatives against pluvial degradation, cannot readily be formed in such mountainous and stormy regions.

Some few lavas in Madeira have a singularly recent aspect as compared to others which are covered with a considerable depth of vegetable soil. I allude particularly to the lava currents near Port Moniz, one of which is as rough and bristling as are some streams before alluded to in Palma (p. 641) of historical date. I am indebted to M. Hartung for the annexed drawing of lava at Port Moniz, which I did

not visit myself. It is traversed by a channel, a, like one of those already described (p. 637). For how long a period such characters may be retained is uncertain, so much does this depend on the mineral composition of the rock. Some of the lavas of Auvergne, of prehistorical date and certainly of high antiquity, are almost as rugged; so that this freshness of aspect is only a probable indication of a relatively modern origin.

CHAPTER XXX.

ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS.

Tests of relative age of volcanic rocks-Tests by superposition and intrusionTest by alteration of rocks in contact-Test by organic remains-Test of age by mineral character-Test by included fragments-Volcanic rocks of the PostPliocene period-Basalt of the Bay of Trezza in Sicily-Post-Pliocene volcanic rocks near Naples-Dikes of Somma.

HAVING referred the sedimentary strata to a long succession of geological periods, we have now to consider how far the volcanic formations can be classed in a similar chronological order. The tests of relative age in this class of rocks are four: 1st, superposition and intrusion, with or without alteration of the rocks in contact; 2d, organic remains; 3d, mineral characters; 4th, included fragments of older rocks.

Tests by Superposition, &c.—If a volcanic rock rest upon an aqueous deposit, the former must be the newest of the two; but the like rule does not hold good where the aqueous formation rests upon the volcanic, for melted matter, rising from below, may penetrate a sedimentary mass without reaching the surface, or may be forced in conformably between two strata, as b at D in the annexed figure (fig. 710), after which it may cool down and consolidate. Superposition,

therefore, is not of the same value as a test of age in the unstratified volcanic rocks as in fossiliferous formations. We can only rely implicitly on this test where the volcanic rocks are contemporaneous, not where they are intrusive. Now, they are said to be contemporaneous if produced by volcanic action which was going on simultaneously with the deposition of the strata with which they are associated. Thus in the section at D (fig. 710), we may perhaps ascertain that the trap b flowed over the fossiliferous bed c, and that, after its consolidation, a was deposited upon it, a and c both belonging to the same geological period. But if the stratum a be altered by b at the point of contact, we must then conclude the trap to have been intru

sive, or if, in pursuing b for some distance, we find at length that it cuts through the stratum a, and then overlies it as at E.

Fig. 711.

We may, however, be easily deceived in supposing the volcanic rock to be intrusive, when in reality it is contemporaneous; for a sheet of lava, as it spreads over the bottom of the sea, cannot rest everywhere upon the same stratum, either because these have been denuded, or because, if newly thrown down, they thin out in certain places, thus allowing the lava to cross their edges. Besides the heavy igneous fluid will often, as it moves along, cut a channel into beds of soft mud and sand. Suppose the submarine lava F (fig. 711) to have come in contact in this manner with the strata, a, b, c, and that after its consolidation the strata, d, e, are thrown down in a nearly horizontal position, yet so as to lie unconformably to F, the appearance of subsequent intrusion will here be complete, although the trap is in fact contemporaneous. We must not, therefore, hastily infer that the rock r is intrusive, unless we find the strata, d, e, or c, to have been altered at their junction, as if by heat.

a

The test of age by superposition is strictly applicable to all stratified volcanic tuffs, according to the rules already explained in the case of other sedimentary deposits (see p. 93).

Test of Age by Organic Remains. We have seen how, in the vicinity of active volcanoes, scoriæ, pumice, fine sand, and fragments of rock are thrown up into the air, and then showered down upon the land, or into neighboring lakes or seas. In the tuffs so formed, shells, corals, or any other durable organic bodies which may happen to be strewed over the bottom of a lake or sea will be imbedded, and thus continue as permanent memorials of the geological period when the volcanic eruption occurred. Tufaceous strata thus formed in the neighborhood of Vesuvius, Etna, Stromboli, and other volcanoes now active in islands or near the sea, may give information of the relative age of these tuffs at some remote future period when the fires of these mountains are extinguished. By evidence of this kind we can establish a coincidence in age between volcanic rocks and the different primary, secondary, and tertiary fossiliferous strata.

The tuffs alluded to may not always be marine, but may include, in some places, freshwater shells; in others, the bones of terrestrial quadrupeds. The diversity of organic remains in formations of this nature is perfectly intelligible, if we reflect on the wide dispersion of ejected matter during late eruptions, such as that of the volcano of Coseguina, in the province of Nicaragua, January 19, 1835. Hot cinders and fine scoria were then cast up to a vast height, and covered the ground as they fell to the depth of more than 10 feet and for a distance of 8 leagues from the crater in a southerly direction. Birds, cattle, and

wild animals were scorched to death in great numbers, and buried in ashes. Some volcanic dust fell at Chiapa, upwards of 1200 miles, not to leeward of the volcano as might have been anticipated, but to windward, a striking proof of a counter-current in the upper region of the atmosphere, and some on Jamaica, about 700 miles distant to the northeast. In the sea, also, at the distance of 1100 miles from the point of eruption, Captain Eden, of the "Conway," sailed 40 miles through floating pumice, among which were some pieces of considerable size.* Test of Age by Mineral Composition.-As sediment of homogeneous composition, when discharged from the mouth of a large river, is often deposited simultaneously over a wide space, so a particular kind of lava flowing from a crater during one eruption, may spread over an extensive area; as in Iceland in 1783, when the melted matter, pouring from Skaptar Jokul, flowed in streams in opposite directions, and caused a continuous mass the extreme points of which were 90 miles distant from each other. This enormous current of lava varied in thickness from 100 feet to 600 feet, and in breadth from that of a narrow river gorge to 15 miles. Now, if such a mass should afterwards be divided into separate fragments by denudation, we might still perhaps identify the detached portions by their similarity in mineral composition. Nevertheless, this test will not always avail the geologist; for, although there is usually a prevailing character in lava emitted during the same eruption, and even in the successive currents flowing from the same volcano, still, in many cases, the different parts even of one lava-stream, or, as before stated, of one continuous mass of trap, vary much in mineral composition and texture.

In Auvergne, the Eifel, and other countries where trachyte and basalt are both present, the trachytic rocks are for the most part older than the basaltic. These rocks do, indeed, sometimes alternate partially, as in the volcano of Mont Dor, in Auvergne; and we have seen that in Madeira trachytic rocks may overlie an older basaltic series (p. 653); but the great mass of trachyte occupies more generally perhaps an inferior position, and is cut through and overflowed by basalt. It can by no means be inferred that trachyte predominated at one period of the earth's history and basalt at another, for we know that trachytic lavas have been formed at many successive periods, and are still emitted from many active craters; but it seems that in each region, where a long series of eruptions have occurred, the more felspathic lavas have been first emitted, and the escape of the more augitic kinds has followed. The hypothesis suggested by Mr. Scrope may, perhaps, afford a solution of this problem. The minerals, he observes, which abound in basalt are of greater specific gravity than those composing the felspathic lavas; thus, for example, hornblende, augite, and olivine are each more than three times the weight of

* Caldcleugh, Phil. Trans., 1836, p. 27.
See Principles, Index, "Skaptar Jokul."

water; whereas common felspar, albite, and Labrador felspar have each scarcely more than 2 times the specific gravity of water; and the difference is increased in consequence of there being much more iron in a metallic state in basalt and greenstone than in trachyte and other felspathic lavas and trap rocks. If, therefore, a large quantity of rock be melted up in the bowels of the earth by volcanic heat, the denser ingredients of the boiling fluid may sink to the bottom, and the lighter remaining above would in that case be first propelled upwards to the surface by the expansive power of gases. Those materials, therefore, which occupy the lowest place in the subterranean reservoir will always be emitted last, and take the uppermost place on the exterior of the earth's crust.

Test by Included Fragments.-We may sometimes discover the relative age of two trap rocks, or of an aqueous deposit and the trap on which it rests, by finding fragments of one included in the other in cases such as those before alluded to, where the evidence of superposition alone would be insufficient. It is also not uncommon to find a conglomerate almost exclusively composed of rolled pebbles of trap, associated with some fossiliferous stratified formation in the neighborhood of massive trap. If the pebbles agree generally in mineral character with the latter, we are then enabled to determine its relative age by knowing that of the fossiliferous strata associated with the conglomerate. The origin of such conglomerates is explained by observing the shingle beaches composed of trap pebbles in modern volcanic islands, or at the base of Etna.

Newer Tertiary Pliocene Periods.—I shall now select examples of contemporaneous volcanic rocks of successive geological periods, to show that igneous causes have been in activity in all past ages of the world, and that they have been ever shifting the places where they have broken out at the earth's surface.

One portion of the lavas, tuffs, and trap-dikes of Etna, Vesuvius, and the island of Ischia has been produced within the historical era; another and a far more considerable part originated at times immediately antecedent, when the waters of the Mediterranean were already inhabited by the existing testacea, but when certain species of elephant, rhinoceros, and other quadrupeds now extinct, inhabited Europe. A third and more ancient portion again of these volcanoes originated at the close of the Newer Pliocene period, when less than ten, sometimes only one, in a hundred of the shells differed from those now living (see p. 190).

It has already been stated that in the case of Etna, Post-pliocene formations occur in the neighborhood of Catania, while the oldest lavas of the great volcano are Pliocene. These are seen associated with sedimentary deposits at Trezza and other places on the southern and eastern flanks of the great cone (see above, p. 190).

The Cyclopian Islands, called by the Sicilians Del Farraglioni, in the sea-cliffs of which these beds of clay, tuff, and associated lava are